Sewage sludge is produced as an unwanted waste product during the purification of sewage in sewage purification plants. It was formerly recovered as manure in agriculture and today this still counts as the simplest and least expensive form of disposal. Agricultural utilization was greatly restricted as a result of the high contents of heavy metals in sewage sludge, particularly in sewage sludge from large communal sewage purification plants. In the meantime, the greater part of the sewage sludge is stored in dumps or burnt in combustion installations. The burning of sewage sludge is very expensive and pollutes the air by the emission of harmful substances. It is true that the dumping of sewage sludge is comparatively simple; but the expenses are constantly rising because the requirements with regard to the safety of depositing in dumps are constantly increasing.
German Offenlegungsschrift 32 21 495 shows a process and a plant of the type mentioned above, wherein sewage-sludge filter cakes coming from chamber filter presses are comminuted in a grinding and drying device and then fed to a rotary kiln for the conversion. This conversion is a pyrolysis process during which a low-temperature carbonization of the sewage sludge takes place with the complete or far-reaching exclusion of air. As a result of a so-called low-temperature pyrolysis, that is to say as a result of a conversion at temperatures around 320.degree. C., gas containing oil and combustible coke are formed from the waste. The oil is extracted from the gas containing oil by condensation. The coke is burnt in a furnace; the hot flue gases produced in the course of this are used to heat the rotary conversion kiln and the grinding and drying device. The known process is not yet satisfactory with regard to the heat recovery, that is to say with regard to the energy balance of the whole process and with regard to the precision of conducting the process. In addition, the known plant is not flexible with regard to different amounts of sewage sludge occurring and having to be processed.